This invention relates to the art of drilling and in particular it relates to wire line core drilling apparatus.
In the course of wire-line drilling, a core barrel inner tube assembly is dropped or pumped along the bore of a drill string to a position just above or behind the drill bit. The drill string is provided with an annular landing shoulder therein. The inner tube assembly is also provided with a landing shoulder which is adapted to co-operate with and seat on the landing shoulder of the drill string. The inner tube assembly is provided with spring loaded latches which automatically move outwardly and engage in an annular recess, termed the latch seat, which is provided in the lower section of the drill string (otherwise known as the outer tube) thereby to anchor the inner tube assembly against axial movement in the drill string. A drilling liquid, typically water, is pumped along the drill string thereby to propel the inner tube assembly along to the landing position which is of course correctly positioned relative to the drill bit.
The primary objective of a core drilling operation is to obtain a core of drilled material for purposes of geological analysis. The distal end of the drill string is accordingly provided with an annular drill bit of any desired well known variety, the bit having diamonds or board embedded therein to enable the bit to cut through the hardest formations likely to be encountered. As the drilling proceeds, the rotating bit cuts through the formations and a core of the formation being drilled moves into and is captured by the core receiving barrel of the inner tube assembly. When the core barrel is filled, the drilling operator on the surface passes an overshot assembly along the drill string. The overshot assembly is arranged to engage with the upper end of the inner tube assembly and a wire line attached to the overshot is then tensioned in the course of which the spring loaded latches release thus allowing the inner tube assembly to be pulled to the surface. The core of material, which has broken off from the formation, is captured within the inner tube assembly in well known fashion and when the inner tube assembly reaches the surface the core is removed and taken away for analysis. Following this, the inner tube assembly is then passed along the drill string in preparation for the taking of a further sample. The flow of drilling liquid is typically provided by a flush pump which is capable of producing the flow rates and pressures required during the course of a drilling operation.
During the course of a normal core drilling operation, the above-noted pump forces the drilling liquid along the drill string, through and along the above-noted inner tube assembly, and to the bit where the liquid cools the bit and flushes away the cuttings therefrom, the fluid velocity being sufficient to move these cuttings along the exterior of the drill string and along the drill hole to the surface.
Surface drilling differs from underground drilling primarily as a result of the orientation or inclination of the holes to be drilled. The average surface bore hole may have an orientation which is anything from a truly vertical position to a position making approximately a 45.degree. angle with the surface. An underground drill hole may be oriented anywhere between a horizontal position and a position which is vertical or straight-up.
In the case of surface drilling, gravity is used to advantage as the core barrel and overshot assembly are being moved to their respective positions and the use of drilling liquid to assist in moving these items is to complement the force of gravity and to speed up the process. However, in underground drilling, the propulsion of the tools hydraulically in an efficient manner is of paramount importance as gravity is working against these tools as they are being moved toward the bit-end of the drill string.
In order that the hydraulic propulsion process may be as efficient as possible, prior art core drilling apparatus has made provision for a type of piston packing on both the core barrel and overshot assemblies made either from rubber or a suitable polymer material. The piston packing typically takes the form of one or more relatively thick washers. With these packings in place on the wire line assemblies, the same can be squeezed axially to match approximately the inside diameter of the drill string by way of an adjusting nut to apply the correct amount of axial pressure. If the piston packing is too loose, a substantial portion of the propelling fluid will bypass the core barrel and/or overshot assembly and the process of hydraulically propelling the latter along the drill string will be slow and inefficient. In theory, these piston packings are supposed to work but in practice the outside diameter of the resilient packing soon becomes reduced as a result of wear and if the piston packings are not constantly adjusted by means of the adjustment nuts noted above, the hydraulic pumping operation soon becomes slow and inefficient.